Patent GB-806562-A (GB)
Title: Improvements in or relating to telephone systems having substation circuits including transistors
IPC: H04M1/76
Filed: 1957-03-08
Abstract: 806,562. Substation circuits. WESTERN ELECTRIC CO. Inc. March 8, 1957 [March 29, 1956], Nc. 7744/57. Class 40 (4). [Also in Group XL (c)] The transmission level of a number of subscribers is maintained constant regardless of loop length by employing a transistor amplifier in each subset, the bias being varied with loop length to alter the gain. A central battery 13, Fig. 1, supplies operating potentials to the collector, base and emitter of transistor 16, a telephone transmitter 15 being connected in series with a resistor 31 in the emitter lead and a circuit independent of the transistor and the device 15 is arranged to supply current from the supply 13 to the junction of the device 15 and resistor 31. The circuit may operate in such a manner that a constant current is drawn from the central battery 13 irrespective of loop lengths 11 by selecting an impedance 27 to provide a constant potential, in which case the impedance 33 is a resistor. Since the device 27 maintains the base potential constant, a similar potential appears across the emitter resistor 31 so that if the loop length is short and the current flowing through resistors 33, 31 relatively large, a correspondingly small current flows in the collector-emitter path of transistor 16 and the amplification of signals from the transmitter 15 is reduced. In an alternative arrangement presenting a constant resistance to the central supply the impedance 27 is a resistor so that the base potential is linearly related to the potential appearing across terminals 10, the impedance 33 then being chosen to vary rapidly with current, operation on a short loop causing an increased current through resistors 33, 31 with the result that the current in transistor 16 is reduced. In a practical circuit, Fig. 2, requiring a constant current from the supply 13 the baseemitter path is biased by a breakdown diode 40 maintained in this condition by current passed through resistor 26. Since on long loops the diode 40 may not break down a by-pass capacitor 20 is arranged to present a low impedance. An isolating resistor 42 is included since the diode 40 becomes very noisy near the breakdown potential. A resistor 41 is shunted across diode 40 to prevent the base-collector potential falling to too low a value. A resistor 43 provides a load into which the transmitter 15 may work and the output impedance of the circuit is lowered by employing a negative feedback resistor 44. The receiver 50 is provided with a sidetone balancing network 51, 52 and 53 and is shunted by a network 54, 55 and 58 the impedance of which increases with loop lengths. The varistors 54, 55 are connected effectively across receiver 50 by capacitors 56, 57 and D.C. flows through resistor 58 dependent on the loop lengths. With a short loop the current is relatively greater and the varistors 54, 55 present a low shunt impedance. In a modification, Fig. 3, which presents a constant resistance to the central office the base is connected to a simple potential divider 26, 61 and the independent circuit feeding resistor 31 includes a breakdown diode 63 and the varistor 62. With a short loop length the current flowing through the auxiliary circuit increases more quickly than that flowing through the base potential divider so that the current flowing in the collector-emitter path of transistor 16 is reduced, and the amplification applied to the signals from telephone transmitter 15 is decreased. Specification 773,850 is referred to.